It's a terrible design. You should always be able to mute all with a single, reliable push on a volume rocker switch. iOS gets this right.
Consider yourself in a quiet place, and then you have to play a movie on your phone. How do you know current volume level? Is it mute or on highest level? Well, to play the video on your Android device with no sound (mute), do as follows:
1. Open Settings and then go to Sounds (If you own a Galaxy device, you just need to drag down the notification bar and long press on the Volume icon)
2. Tap Volume
3. Make sure Music, Video, Games and other media level sound is at its minimum level (mute)
4. Now, you can start playing the video with no worries